I'm really surprised at the number of people who don't have alternative (non-family) child care available. I have more backups for people to watch my dogs than many people seem to have for their children!
It's easy to rely on family alone... until there's something that comes up and makes them unavailable. In this case, it's a happy event, and if the OP doesn't go, no real harm done.
But what if the "usual" family baby sitter is in a car accident or becomes seriously ill at the same time that there's a crisis in one's immediate family? What if the "usual" family baby sitter is off on a cruise or vacation far away and there's a disaster at home and no place to put the kids while you deal with it?
I wouldn't have strangers come to my hotel room to watch my kids (even at WDW) but the way I grew up families who traveled took teenage cousins or family friends with them to places when they would need evening/special occasion child care. Or they left them home with friends or neighbors and repaid the favor when THEY went away.